Specialized transport proteins in the lipid bilayer perform the translocation of solutes across biological membranes. The prokaryotic L-carnitine/ -butyrobetaine transporter CaiT is a member of the betaine/carnitine/choline transporter (BCCT) family. However, CaiT is an interesting exception within the BCCT family since the transporter functions as a Na+- and H+- independent antiporter, while most members of the BCCT family require either an additional sodium ion or a proton to transport substrates into the cell. The three-dimensional X-ray crystal structure of CaiT reveals two different...